1. Field of the Invention
This invention relates to a process for combustion of hydrocarbonaceous fuels containing high levels of sulfur, fuel-bound nitrogen, and low levels of ash which inhibits the emissions of solid wastes, oxides of nitrogen and sulfur.
2. Description of the Prior Art
In conventional processes for the combustion of hydrocarbonaceous fuels containing high levels of sulfur, fuel-bound nitrogen, and low levels of ash utilizing sorbent injection, the production of solid waste streams containing calcium sulfate, unspent sorbent and ash is disproportionately large. For example, the combustion of petroleum coke, which contains approximately 1.5% fuel-bound nitrogen, 4% sulfur and 0.2% ash using a known combustion process with sorbent injection for sulfur removal, consumes about 250 lbs. of calcium-based sorbent for every 1,000 lbs. of coke combusted and produces a solid waste stream in excess of 240 lbs., more than half of which is in the form of calcium sulfate, a very stable product which is difficult to regenerate and reuse. In addition, in this known single stage combustion process, a significant portion of the fuel-bound nitrogen is converted into oxides of nitrogen which are environmentally unacceptable.
Several processes for the combustion of sulfur-and nitrogen-containing carbonaceous materials are disclosed by the prior art. U.S. Pat. No. 4,854,249 discloses a process for combustion of sulfur- and nitrogen-containing carbonaceous fuels in a two stage combustion process in which sulfur- and nitrogen-containing carbonaceous materials and a sulfur fixation agent are introduced into a first combustion stage of a fluidized bed having a density/size selective solids withdrawal conduit at the base of the fluidized bed. Gaseous sulfur compounds are reacted with oxides of the sulfur fixation agent in a reducing region of the fluidized bed to form an intermediate solid metallic sulfur compound which, in turn, is reacted in an oxidizing region at the base of the fluidized bed to form a sulfur-containing compound which is withdrawn through the density/size selective solids withdrawal conduit together with agglomerated ash. Nitrogen contained in the carbonaceous materials is converted to molecular nitrogen in the reducing region of the fluidized bed. Ash produced in the bed is agglomerated and withdrawn through the density/size selective solids withdrawal conduit.
U.S. Pat. No. 4,848,251 teaches a process for combustion of carbonaceous materials containing ash and sulfur in slagging combustors, including control of slag within a temperature range of 2000.degree. F.-2500.degree. F., slag stoichiometry less than oxiding, and gas phase stoichiometry near stoichiometric. Other known methods for reducing sulfur emissions in combustion processes are disclosed by U.S. Pat. No. 4,651,653 which teaches a sorbent injection system in which a sorbent, such as limestone, is introduced into a furnace in which a sulfur-bearing fuel is burned at gas temperatures of approximately 2400.degree. F.; U.S. Pat. No. 4,824,360 in which fuel and oxidizer are combusted in a combustion reactor at temperatures between 900.degree. C.-1500.degree. C., the combustion gases formed being directed into a suspension reactor into which a sulfur fixation compound is fed; U.S. Pat. No. 4,824,441 in which a composition of refined coal having low ash forming material and inorganic sulfur content, a sulfur sorbent, a sulfation promoter, and a catalyst for the reaction of sulfur dioxide to sulfur trioxide is burned in an oxygen restricted burner; U.S. Pat. No. 4,872,423 in which the effectiveness of sulfur absorbent when burning sulfur-containing coal in a fluidized bed of particulate material is improved by removing the bed material containing sulfur absorbent from the fluidized bed and mixing it with steam; U.S. Pat. No. 4,913,068 in which the effectiveness of a sulfur absorbent containing calcium in a power plant burning fuel in a fluidized bed of particulate matter is improved by crushing the absorbent and dividing it into fine and coarse fractions, supplying the course fraction directly to the fluidized bed, and calcinating the fine fraction which, acting as a drying agent for the fuel, is introduced into the bed together with the fuel; U.S. Pat. No. 4,936,047 in which a mixture of sulfur-containing particulate fuel and sulfur absorbent is injected into a reducing atmosphere in a combustor at a temperature of at least 1500.degree. F., forming a gaseous portion and a solid portion, the solid portion being further combusted, forming an ash containing sulfur fixed therein; and U.S. Pat. No. 4,981,667 in which the sulfur content of flue gases from the combustion of petroleum pitch, containing about 40% petroleum coke dissolved therein, and petroleum pitch, containing about 25% sawdust dissolved therein, is converted into a solid by adding lime to the fuel before burning it.
Fines recirculation in a fluidized bed combustor is disclosed in U.S. Pat. No. 4,843,981 in which a bed containing inert particles, ash and some partially burned solid fuel particles on a distribution plate within an internal combustion chamber of a fluidized bed combustor are fluidized by a fluidizing gas flowing upward within the combustion chamber from below the distribution plate and having a velocity such that fine particles are carried upward within the combustion chamber, captured in a particulate recycle separator and, subsequently, returned to the fluidizing bed. Fluidizing gas passing through the fines separator continues through a convective heat exchanger in which heat is removed from the gas and, downstream of the convective heat exchanger, through a filter system to remove dust before being exhausted to the atmosphere U.S. Pat. No. 4,704,084 discloses a method of lowering nitrogen oxides and minimizing sulfur dioxide in the reaction gases from the combustion of fuel in a multi-solid fluidized bed. The lower region of the fluidized bed is operated under substoichiometric conditions. The upper region of the fluidized bed is operated under oxiding conditions. A size difference is maintained between large and small particles in the bed such that substantially all of the larger particles are at least four times the size of the smaller particles. A portion of the fine particles is recycled to the lower region of the bed and another portion of the fine particles is recycled to the upper region of the bed.
U.S. Pat. No. 4,960,057 discloses a method of waste incineration using a fluidized bed type incinerator in which a granular material comprising titanium oxide or aluminum oxide is employed as a fluidizing medium to prevent the generation of harmful substances such as dioxin.
In all the known processes for the combustion of carbonaceous fuels containing fuel-bound nitrogen, sulfur and low levels of ash, the solid waste produced from such processes cannot be recycled. Rather, elaborate discharge mechanisms and processes to stabilize the waste are required.